This invention is in the field of motor control, and is more specifically directed to control of a voice coil motor in a disk drive system.
Magnetic disk drive technology is the predominant mass non-volatile storage technology in modern personal computer systems, and continues to be an important storage technology for mass storage applications in other devices, such as portable digital audio players. As is fundamental in the field of magnetic disk drives, data is written by magnetizing a location (“domain”) of a layer of ferromagnetic material disposed at the surface of a disk platter. Each magnetized domain forms a magnetic dipole, with the stored data value corresponding to the orientation of that dipole. The “writing” of a data bit to a domain is typically accomplished by applying a current to a small electromagnet coil disposed physically near the magnetic disk, with the polarity of the current through the coil determining the orientation of the induced magnetic dipole, and thus the data state written to the disk. In modern disk drives, a magneto-resistive element is used to sense the orientation of the magnetic dipole at selected locations of the disk surface, thus reading the stored data state. Typically, the write coil and the magneto-resistive element are physically placed within a read/write “head”.
In conventional disk drive systems, a spindle motor rotates the disk platters, and a “voice coil” motor moves an actuator arm on which the read/write heads are mounted, at a distal end from the motor. The voice coil motor thus moves the read/write heads to the track of the disk surface corresponding to the desired address. Conventional voice coil motors are servo-controlled, in that position indicators on the disk surface are sensed, and fed back to a control loop for adjustment of the position of the actuator arm to the desired location. Typically, an “outer” control loop compares the feedback position signal to the desired position of the actuator, and a torque command value is derived from a difference in these values. The torque command is used to produce a drive current to the voice coil motor that produces a motor torque to move the actuator arm in the desired direction. In conventional voice coil motor control circuits, an “inner” control loop is also included, in which the current applied to the motor is sensed, and applied to a feedback loop for control of this motor current.
FIG. 1 illustrates a conventional drive circuit for a voice coil motor in a disk drive system. VCM digital-to-analog converter (DAC) 22 receives a digital torque command signal TRQ_CMD from the “outer” servo control loop for the voice coil motor, and produces an analog signal via resistor 23 to the negative input of error amplifier 24, which receives reference voltage VREF at its positive input. The single-ended output of error amplifier 24 is applied to power amplifier 28, which generates differential output current iOUT at its output terminals T3 and T4. This output current iOUT is applied to motor M, which presents an impedance to power amplifier 28 in the form of an inductor Lm and a parasitic resistor Rm. Sense resistor 31 is included in the loop with motor M, and the voltage across this resistor 31 is sensed by sense amplifier 32, at terminals T5, T6. The output of sense amplifier 32 is applied to a summing node at the output of VCM DAC, via resistor 29. As such, the voltage across sense resistor 31 is applied as negative feedback to the desired torque signal at the output of DAC 22. When the circuit is in balance (i.e., current iOUT is equal to the desired current corresponding to the torque command signal TRQ_CMD), the voltage at the negative input of error amplifier 24 will equal reference voltage VREF.
As fundamental in the control system art, the impedance presented by motor M determines the response characteristic of this inner control loop. Specifically, the inductance Lm of motor M defines a “pole” in the frequency response of this loop, such that the response of the system varies with frequency, and defines a frequency at which oscillation can occur. As known in the art, compensation for the frequency response of the motor can be implemented into the control loop, so that the frequency response of the control loop can meet system requirements, and to avoid instability in operation. In the conventional circuit of FIG. 1, such compensation is realized by way of an R-C network connected across error amplifier 24. In this case, the compensation network includes capacitor 25 connected in parallel with the series network of resistor 26 and capacitor 27. The component values of capacitors 25, 27 and resistor 26 are based on the inductance Lm and resistance Rm of motor M, and on the desired bandwidth (i.e., frequency range over which the control loop has adequate response) for the operating frequencies of the control loop.
As well known in the control systems art, capacitor 27 serves as an integrator in the control loop, which provides higher gain at lower frequencies in the frequency response of the control loop. Resistor 26 flattens the response characteristic, to counteract the 90° phase shift (and corresponding reduced phase margin) inserted into the control loop response by integrating capacitor 27, improving stability. Capacitor 25 rolls off the response at high frequency, beyond the desired cutoff frequency.
As shown in FIG. 1, VCM DAC 22, error amplifier 24, power amplifier 28, and sense amplifier 32 (as well as passive components such as resistors 23, 25) are implemented into a single integrated circuit 20. For voice coil motor M of conventional size and impedance, the component values required for the compensation network of capacitors 25, 27 and resistor 26 is typically realized “off-chip”, external to integrated circuit 20. As shown in FIG. 1, this compensation network is connected across terminals T1 and T2 of integrated circuit 20, which are connected to the input and output of error amplifier 24 as shown. Because these components are connected externally, their values can be selected with high precision, providing precise compensation. Conversely, if capacitors 25, 27 and resistor 26 were implemented “on-chip”, their component values could vary by as much as 10-20%, which is not tolerable for reasonable compensation of modern voice coil motor control loops.
This conventional voice coil control circuit shown in FIG. 1 is referred to in the art as a “single-ended” control loop, in that the drive current is derived from a single-ended signal at the output of error amplifier 24, driving a single-ended input to pseudo-differential power amplifier 28. As such, the external compensation network can be realized by three external components across two external terminals of integrated circuit 20.
However, many technology trends now favor the use of a fully differential power stage to drive the voice coil motor in modern disk drive systems. The storage density of disk drives (measured in tracks per inch) continues to increase, which requires improved noise rejection capability for the voice coil motor control loop. In addition, the power supply voltage of disk drive controller electronics is also trending lower with continued miniaturization of disk drive systems and with the trend toward battery power, for example in disk drive-based portable digital audio players. This lower power supply voltage level reduces the linear swing “head room” of the control circuit. The use of a fully differential control loop (e.g., a differential error amplifier driving a differential power amplifier) will provide the desired rejection of noise coupling from the power supply, substrate, or other circuit functions, will decrease the total harmonic distortion in the control loop, and also requires one-half the linear swing “head room” of that required for single-ended motor drive as shown in FIG. 1.
However, implementation of conventional external “off-chip” compensation for a fully differential control circuit requires two sets of compensation components. Referring to FIG. 1, two instances of the parallel compensation network of capacitors 25, 27 and resistor 26 will be required for a fully differential stage. This of course requires twice the number of external components, and requires two additional integrated circuit terminal pins, as compared with the single-ended case. The additional expense in components and, more significantly, in pin count and circuit board space, can be prohibitive, especially in miniaturized systems such as digital audio players.